In this context, three major problems arise:
the bandwidth of the carrier can be less than the frequency uncertainties of the satellite link. These uncertainties can originate in various ways: low relative stability of the local oscillators in the frequency transposition equipment, the presence of a Doppler effect, satellite drift, etc. Consequently, it must be possible to accommodate a large frequency difference between the received carrier and the local demodulation oscillator; PA1 the carrier may be absent or present (transmission in SCPC mode); and PA1 carrier detection must take place quickly (particularly with DTMA transmission) and reliably, in spite of a bit energy/noise (Eb/No) ratio close to 2 dB without information coding. PA1 a timing estimator responding to the baseband samples to provide an error signal corresponding to the phase error between the clock frequency that is to be recovered and the local clock frequency; and PA1 detection means generating a detection signal indicating that the carrier wave has been detected, whenever a level representative of the variance of the error signals reaches a threshold value.
More precisely, the detection system of the invention must be capable of being integrated in a receiver that is entirely digital, with detection taking place prior to demodulation, and in spite of unfavorable noise conditions (Eb/No=2 dB). Also, detection must be capable of being performed quickly, with a maximum duration in the vicinity of 100 symbol times.
A system is known for detecting the presence or the absence of a carrier wave based on establishing a phase histogram of the sampled signal. Detecting non-uniformity in the histogram indicates the presence of a carrier wave.
FIG. 1 shows the constellation of a 4-PSK type signal as received, together with the observed phases .theta..sub.i. The axes representing the carriers in phase quadrature are referenced P and Q. On each new received symbol (assuming that the clock rate has already been recovered), the difference .DELTA..theta..sub.i =(.theta..sub.i -.theta..sub.i-1) modulo .+-..pi./4 is calculated. On the basis of the observed phase differences .DELTA..theta..sub.i, a histogram of phase differences is constructed as shown in FIG. 2. In this case, the histogram has four classes (4-PSK) and is obtained by incrementing the class corresponding to .DELTA..theta. and by decrementing by the same amount the class .DELTA..theta..sub.k-N where N is the analysis window expressed as a number of symbols. In the presence of a signal that is actually being received, as opposed to in the presence of noise, the accumulated phase differences .DELTA..theta..sub.i give rise to a difference D (where D=.SIGMA..DELTA..theta..sub.max -.SIGMA..DELTA..theta..sub.min) greater than a predefined threshold value.
The drawback of that method is that in the presence of noise or a high level of frequency drift, phase differences are observed to become more uniform and the threshold value is not reached, in spite of a carrier being present. If the threshold value is made smaller, than the false alarm rate becomes too great. Consequently, that method cannot be applied to signals suffering from large amounts of frequency drift between the carrier frequency and the demodulation frequency, or to signals suffering from a large amount of noise.
In addition, that method assumes that the clock has already been acquired, and that is expensive in time.